Paper SS2-WeM4
Identification of C₂H_x Intermediates on Pt(111) through the Combined Use of Low Temperature Scanning Tunneling Microscopy and Reflection Absorption Infrared Spectroscopy

Wednesday, November 11, 2009, 9:00 am, Room N

Various C₂H_x surfaces intermediates formed through the thermal decomposition of acetylene and ethylene on the Pt(111) surface were identified and their stabilities characterized through complementary studies using the techniques of low temperature scanning tunneling microscopy (LT-STM) and reflection absorption infrared spectroscopy (RAIRS). By providing high-resolution vibrational spectra of surface species, RAIRS data is highly sensitive to the chemical identity of those species. However, it does not provide information on the relative coverages of adsorbed molecules and many important stable intermediates are invisible to RAIRS. In contrast, with a LT-STM operated at 4.7 K, individual molecules are observable and their absolute coverages are readily obtained simply by counting, yet their chemical identities cannot be directly determined from the STM images alone. In the case of acetylene, both RAIRS and LT-STM indicate that it adsorbs in a single form at low temperature in which the CC bond is positioned over a three-fold hollow site, and that the interaction with the surface occurs through both di-σ and π bonding. On warming to 250 K in the presence of coadsorbed hydrogen, RAIRS provides strong evidence for the partial hydrogenation of acetylene to form an adsorbed vinyl (CHCH₂) species. The LT-STM images following a 250 K anneal in the presence of coadsorbed hydrogen show a marked increase in the coverage of a species that is identified as vinyl, based on the RAIRS results. Both techniques indicate that adsorbed acetylene is stable up to 300 K. A third species, in addition to adsorbed acetylene and vinyl, is also observed with the LT-STM after a 300 K anneal, which is identified as vinylidene, CCH₂. However, the RAIRS evidence for a vinylidene species is not definitive. In the case of ethylene adsorption at low temperature, RAIRS provides clear evidence for a di-σ bonded form of ethylene at low temperatures that coverts to ethylidyne at 280-420 K. The LT-STM images show that ethylene exists in both π-bonded and di-σ bonded forms at low temperature and that the two forms can be easily interconverted using electron pulses from the STM tip. On the basis of RAIRS experiments, it was found that surface carbon formed through the complete dehydrogenation of acetylene could be hydrogenated to form ethylidyne (CCH₃), from which it was inferred that the surface carbon was in the form of C₂ molecules. A form of surface carbon that can be hydrogenated to ethylidyne was also identified with LT-STM. Other surface species that have been investigated with the two techniques include ethynyl (CCH), ethylidene (CHCH₃), and methylidyne (CH).